To improve the common mode rejection ratio of the differential amplifier, the choice of resistors is critical.

Ideally, the resistors in a differential amplifier circuit should be carefully chosen and have the same ratio (R2/R1 = R4/R3). Any deviation from these ratios will result in an undesirable common-mode error. The ability of a differential amplifier to reject this common-mode error is expressed as the common-mode rejection ratio (CMRR). It indicates how the output voltage changes with the same input voltage (common-mode voltage). In the best case, the output voltage should not change because it depends only on the difference between the two input voltages (maximum CMRR); however, this is not the case in practice. CMRR is an important characteristic of a differential amplifier circuit and is usually expressed in dB.

For the differential amplifier circuit shown in Figure 1, the CMRR depends on the amplifier itself and on the externally connected resistors. For the latter, the CMRR that depends on the resistor is denoted by the subscript "R" in the rest of this article and is calculated using the following formula:

For example, in an amplifier circuit, the desired gain G = 1 and using 1% tolerance, 2% matched resistors yields a common-mode rejection ratio of

At 34 dB, the CMRR _R is relatively low. In this case, even if the amplifier has very good CMRR, high accuracy cannot be achieved, because the accuracy of a chain is always determined by its worst link. Therefore, for a precise measurement circuit, the resistors must be chosen very accurately.

The resistance values ​​of conventional resistors are not constant in practice. They are affected by mechanical load and temperature. Depending on the requirements, resistors with different tolerances or matched resistor pairs (or networks) are used, most of which are manufactured using thin film technology and have precise ratio stability. With these matched resistor networks, such as the LT5400 quad matched resistor network, the overall CMRR of the amplifier circuit can be significantly improved. The LT5400 resistor network has excellent matching over the entire temperature range, and even better matching when used in combination with a differential amplifier circuit, thus ensuring a CMRR that is twice as good as that of discrete resistors.

The LT5400 provides 0.005% matching accuracy, resulting in a CMRR _R of 86 dB. However, the total mode rejection ratio (CMRR _{Total )} of the amplifier circuit is composed of the combination of the resistor CMRR _R and the operational amplifier common mode rejection ratio CMRR _OP . For a differential amplifier, this can be calculated using Equation 3:

For example, the LT1468 provides a typical CMRR _OP of 112 dB, and with a gain of G = 1 using the LT5400, its CMRR _Total is 85.6 dB.

Alternatively, an integrated differential amplifier such as the LTC6363 can be used. This amplifier has the amplifier and optimally matched resistors built into a single chip. It virtually eliminates all of the above issues and also provides maximum accuracy, with CMRR values ​​of over 90 dB.

The external resistor circuit must be carefully selected based on the accuracy requirements of the differential amplifier circuit in order to achieve high performance of the system. Alternatively, an integrated differential amplifier can be used, such as the LTC6363, which integrates matched resistors in a single chip.